Loudspeakers that employ bending mode vibrations of a diaphragm or plate to reproduce sound were first proposed at least 90 years ago. The design concept reappeared in the 1960's when it was commercialized as the “Natural Sound Loudspeaker,” a trapezoidal shaped, resin-Styrofoam composite diaphragm structure driven at a central point by a dynamic force transducer. In the description of that device, the inventors identified the “multi-resonance” properties of the diaphragm and emphasized that the presence of higher-order modes increased the efficiency of sound production. The Natural Sound Loudspeaker was employed in musical instrument and hi-fi speakers marketed by Yamaha, Fender, and others but it is rare to find surviving examples today. Similar planar loudspeaker designs were patented around the same time by Bertagni and marketed by Bertagni Electroacoustical Systems (BES).
The basic concept of generating sound from bending waves in plates was revisited by New Transducers Limited in the late 1990's and named the “Distributed-Mode Loudspeaker” (DML). Further research on the mechanics, acoustics, and psychoacoustics of vibrating plate loudspeakers illuminated many of the issues of such designs and provided design tools for the further development of the technology, which remains commercially available from Redux Sound and Touch, a descendant of the original New Transducers Limited by Sonance, which can be traced back to the original BES Corporation in the 1970's, and by others including Tectonic Audio Labs and Clearview Audio.
One physical feature of vibrating panel loudspeakers is the presence of a multiplicity of under-damped mechanical modes of vibration. In contrast, a pistonic loudspeaker can have a single degree of freedom and can be heavily damped, which makes its dynamic response simple in comparison to that of vibrating panel loudspeakers. To address this, panel loudspeaker designs employing wood-polymer composite structures to reduce the ring-down time of excited panel bending modes have been described. Without careful mechanical design measures, the presence of under-damped bending modes in panel loudspeakers can degrade audio quality.
Therefore, what are needed are devices, systems and methods that overcome challenges in the present art, some of which are described above.